Babies are born with a stepping reflex, a biological imperative to push down with their feet and straighten their legs. The stepping reflex is a precursor to walking. By the time they are six months old those movements are becoming more refined and they want to practice those skills all the time. That is also the age at which we start to feed them solid foods.
Traditional high chairs place a baby in a seated position, where the baby's legs are forced in a direction perpendicular to the baby's upper body. Because babies want to stand, step, and move all the time, the rigid, seated position is in direct conflict with the developmental needs of the baby, causing stress, anxiety, and a difficult feeding/eating relationship.
There is increasing awareness of the importance of a high chair to a child's development. Mealtime is where a small child begins to develop language skills. High chair time allows them to join in family social interaction. And since movement spurs their mental and physical growth, both sitting and the inability to move hamper an infant's development.